I keep a couple of dupe detection programs around, I have not used them in a while.

DupeDetector http://www.prismaticsoftware.com/
Picture clones, Percent matching , 2 parameters adjustable to fine tune. Many options for the type of alogrythm used. For finding a duplicate Picture, in one or 2 pools of files. 3 part process that takes some extra thought but can provide for nice listing to cross referance. Dupe logging, auto moving, and other features that I do not use, that may be more usefull to you?

"CloneSpy" http://www.clonespy.com/
Data clones, 100% matching , name matching , compare pools, Good to find repeats that are named different, Not really usefull when the files would have different headers.

After some research, I found that TGA files have no metadata,so that I can use a simple fc to compare them. However, fc is really slow if there are a lot of differences, so I did some searching and found FCMP ( http://www.gammadyne.com/cmdline.htm#fcmp ) which does exactly what I need, for free.

I can also use another command line program to change from other formats to TGA, so that will work well too.